A conventional head-up display system for vehicles includes a projection imaging unit and a curved mirror. The projection imaging unit is disposed in an upper region of a cabin of the vehicle. The curved mirror is disposed on a top surface of an instrument panel of the vehicle. Light rays emitted by the projection imaging unit are projected onto the curved mirror. Light rays reflected from the curved mirror are projected onto a windshield of the vehicle above the instrument panel to form a virtual image in front of the windshield. The virtual image is formed within a line of sight of a driver. While the driver pays attention to traffic, the driver can acquire information of driving from the virtual image at the same time. However, ambient light (e.g., sun light) may be projected on the curved mirror and then be reflected to the windshield, which causes glare to the driver.